1.Field of the Invention
This invention relates to a method of controlling an industrial robot and, more particularly, to an industrial robot arc control method for subjecting the position of a working member to circular-arc control by interpolation while controlling the target angle of the working member with respect to a surface to be worked, where the working member is mounted on the wrist of an industrial robot.
2. Description of the Related Art
Industrial robots have found extensive use in recent years and are capable of performing increasingly sophisticated operations. These industrial robots have the capability to carry out a variety of tasks depending upon the kind of working member mounted at the distal end of the wrist thereof. Control of the robot differs depending upon the kind of task.
FIG. 1 is a view showing the construction of a common industrial robot. The illustrated robot is an articulated robot with movement along five axes. More specifically, the industrial robot depicted in FIG. 1 comprises a base BS which rotates about an axis E, a body BD which rotates about an axis D with respect to the base BS, an arm ARM which rotates about an axis C with respect to the body BD, and a wrist HD which rotates about an axis B with respect to the arm ARM, and which further rotates about an axis A. The robot is therefore an articulated robot having three fundamental axes and two wrist axes, for a total of five axes. An industrial robot of this kind controls the position and travelling velocity of the wrist HD by effecting control along the five axes, and performs a desired task using a working member mounted on the wrist HD. The kind of task and the type of control differ depending upon the type of working member, such as a hand or torch, mounted on the wrist HD. By way of example, an industrial robot for arc welding has a torch mounted at the tip of the wrist HD to serve as the working member, and the surface of a workpiece to be worked is subjected to arc welding along a desired path by means of the torch. In arc welding of this kind or in an operation such as gas cutting, there is the danger of a non-uniform welding or cutting operation unless the angle (target angle) of the torch or working member with respect to the workpiece surface is set to an optimum value. The robot therefore requires that the absolute angle of the torch be controlled by the wrist HD in dependence upon the inclination of the workpiece surface.
More specifically, the robot wrist HD has the rotational axis B with respect to the arm ARM, as well as the rotational axis A for the wrist itself, as shown in FIG. 2(A). When a torch TC is mounted at the distal end of the wrist as shown in FIG. 2(B), the target angle of the torch TC is capable of being varied to assume values of .beta..sub.1, .beta..sub.2 and .beta..sub.3 by rotating the wrist about the axis B.
In a case where the workpiece has an arcuate shape, it is necessary to move the tip of the torch along a circular arc. Consequently, in the prior art, as shown in the explanatory view of FIG. 3, the positions of three points P1, P2 and P3 of the torch tip with respect to the arcuate surface of a workpiece WK to be worked are taught with the target angle .beta..sub.1 being held constant, an interpolated point Pn is obtained from the taught points P1, P2 and P3 by an interpolation method, and arc travel control is performed by controlling the robot about each of its axes of motion in such a manner that the torch tip reaches the interpolated point Pn.
With such arc travel control, however, it is difficult to control the target angle of the torch TC with respect to the work surface, the target angle cannot be held constant during circular interpolation and cannot be varied at each of the taught points. Moreover, according to such conventional control, smooth and continuous control of the target angle of the torch TC with respect to the work surface is difficult to carry out.